Process for improving aluminum silicon alloys



United States Patent 3,151,980 PROCESS FOR IMPROVING ALUMINUM SILICON ALLOYS Kurt Anderko, Frankfurt am Main, and Josef Dornauf,

Kelkheim, Taunus, Germany, assignors to Metallgesellschaft Aktiengesellschaft, Frankfurt am Main, Germany No Drawing. Filed Jan. 10, 1962, Ser. No. 165,292 Claims priority, application Germany, Jan. 19, 1961, M 47,735 3 Claims. (Cl. 75--148) The present invention relates to improvements in a process for improving the grain structure of aluminum silicon alloys by contacting such alloys in the molten state with salts which yield sodium.

It had been known for a long time that the grain structure of aluminum silicon alloys and especially those of eutectic or substantially eutectic composition could be modified to produce a finer grain by adding sodium salts capable of yielding sodium or salt mixtures containing such sodium salts to the aluminum alloys in the molten state. Such procedure for improving the grain structure of aluminum alloys and particularly aluminum silicon alloys has, for example, been described in US. Patent No. 2,013,926. Especially salt mixtures containing sodium fluoride asfthe sodium yielding salt have become known. In practice such salt mixtures contain a relatively large quantity of sodium fluoride in order that the amount of salt mixture required to be added be kept as low as possible. In general these salt mixtures form a liquid phase upon the alloy melt which in the course of time attacks the crucible, particularly in its upper portion at about the height of the upper surface of the molten metal.

It furthermore is known that sodium yielding salts can be introduced into the aluminum-silicon melt aluminothermically or magnesiothermically by mixing the salt mixtures with powdered aluminum or magnesium and immersing such mixture into the melt. While this procedure avoids an attack on the crucible there nevertheless is the danger that the reaction products do not separate out completely in the dross and therefore form inclusions in the metal.

According to the invention it was unexpectedly found that the advantages of both aforementioned processes could be combined while simultaneously avoiding their disadvantages. This is accomplished according to the invention by employing a salt mixture in which a mixture of sodium fluoride and sodium carbonate is employed as sodium yielding substances and which also contains a mixture of sodium chloride and potassium chloride and introducing such mixture together with finely divided magnesium, such as magnesium powder, magnesium shavings or magnesium granules, into the molten aluminum silicon alloy. Aluminum in finely divided form may also be employed along with or in place of the magnesium. According to the invention the quantity of such mixture added to the aluminum silicon alloy melt is 0.2 to 0.7% by weight and such mixtures consist essentially of 50 to 75% of a mixture (1) of 20-40% of finely divided magnesium and/or aluminum, -40% of sodium fluoride and 70% of sodium carbonate and 50 to of a mixture (2) of -60% of sodium chloride and 40-70% of potassium chloride. Preferably, 04-06% of a mixture essentially consisting of 55 to 65% of a mixture (1) of 25-30% finely divided magnesium and/ or aluminum, 10-20% of sodium fluoride and 50-65% of sodium carbonate and to 45% of a mixture (2) of 50% of sodium chloride and 5'060% of potassium chloride is employed. The mixture which is used according to the invention is expediently compressed to form compacts and such compacts introduced into the alumi num silicon alloy melt with the aid of an immersion bell in a manner known per se.

The process according to the invention and the salt mixture employed therefore are characterized by the fact that a mixture of sodium fluoride and sodium carbonate can be employed. Whereas when sodium fluoride is employed as the only sodium yielding salt, 0.3% of a mixture of about 30% of magnesium and 70% of sodium fluoride is required to effect the desired improvement of the alloy and when sodium carbonate is employed as the only sodium yielding salt 0.6% of the corresponding mixture of magnesium and carbonate are required, it was found that when a mixture of of sodium carbonate and 20% of sodium fluoride is employed instead of the individual salts only 0.3% of the corresponding mixture with magnesium was necessary. As a consequence, a substantial portion of the sodium fluoride can be replaced with sodium carbonate without it being necessary to increase the total amount of the salt mixture yielding sodium to effect the desired improvement of the aluminum silicon alloys.

According to the invention a mixture of sodium chloride and potassium chloride is added to the aluminum silicon alloy melt along with mixtures of salts yielding sodium in order to regulate the progress of the reaction in such a way that it is not too vigorous as otherwise the utilization of the mixture would be economically ineflident and an undesired motion of the melt would occur. Also, it was found that the reaction products are not suspended in the metal bath. Both eflects favorably affect the structure and strengths of the treated improved alloys.

The finely divided magnesium and/ or aluminum which is introduced into the molten alloy together with the salt mixture can be in the form of a powder but turnings or granules have also proved suitable. The magnesium and/or aluminum can also be employed as alloys or compounds as long as the required reactivity is maintained as is, for example, the case with the magnesium compound CaMg In general, the use of magnesium has been found more suitable than the use of aluminum.

The following examples will serve to illustrate several embodiments of the process according to the invention.

Example 1 g. of compressed tablets of a mixture of 3 parts by weight of a mixture (1) of 28% of magnesium shavings and 72% of a mixture of 20% of NaF+80% of Na CO and 2 parts by weight of a mixture (2) of 45% of NaCl-|-55% of KCl were prepared and introduced into a melt of 25 kg. of an aluminum silicon alloy containing 11.9% of silicon and 0.37% of magnesium and in addition contained the usual iron and manganese impurities. The amount of the tablets introduced by the 125 g. thereof was 0.5% by weight of the molten alloy.

Test rods were then cast in sand molds. from the thus treated alloy. The rods were given a heat treatment for 4 hours at 530 C. and then quenched in water and then aged for 20 hours at 150 C. and subsequently cooled in air. The thus treated rods exhibited the following properties:

Yield point kg./mm. 25.8 Tensile strength kg./mm. 30.6 Elongation percent 1.2

The values given are each the mean value obtained with 10 rods.

Example 2 kg. of a molten aluminum silicon alloy containing 11.7% of silicon and the usual iron and magnesium impurities were treated as in Example 1 with 0.5% of a 3 mixture also as in Example 1 except that mixture 1 contained 30% of Mg shavings instead of 28% and a correspondingly lower quantity of NaF/Na CO Test rods cast in sand molds but without subsequent treatment possessed the following properties:

Yield point kg./mm. 8.7 Tensile strength kg./mm. 18.7 Elongation percent 14 The values given are each the mean value obtained with 4 rods.

We claim:

1. In a process for improving the grain structure of an aluminum-silicon alloy with the aid of sodium yielding salt mixtures which comprises adding to a molten bath of such alloy 0.2 to 0.7% by weight of a mixture essentially consisting of 50 to 75% of a mixture (1) of 20 to 40% of a finely divided metallic component selected from the group consisting of at least one of the metals magnesium and aluminum, to 40% of sodium fluoride and 20 to 70% of sodium carbonate, and 25 to 50% of a mixture ('2) of 30 to 60% of sodium chloride and 40 to 70% of potassium chloride.

2. In a process for improving the grain structure of an aluminum-silicon alloy with the aid of sodium yielding salt mixtures which comprises adding to a molten bath of such alloy 0.4 to 0.6% by weight of a mixture essentially consisting of 55 to 65% of a mixture (1) of to of a finely divided metallic component selected from the group consisting of at least one of the metals magnesium and aluminum, 10 to 20% of sodium fluoride and '50 to 65% of sodium carbonate, and to of a mixture (2) of 40 to of sodium chloride and 50 to of potassium chloride.

3. The process of claim 1 in which said mixture is added to the molten bath by immersing compacts thereof in such bath.

References Cited in the file of this patent UNITED STATES PATENTS 1,562,654 Pacz Nov. 24, 1925 1,596,888 Pacz Aug. 24, 1926 2,848,321 Bunbury et al Aug. 19, 1958 FOREIGN PATENTS 380,577 Great Britain Sept. 22, 1932 

1. IN A PROCESS FOR IMPROVING THE GRAIN STRUCTURE OF AN ALUMINUM-SILICON ALLOY WITH THE AID OF SODIUM YIELDING SALT MIXTURES WHICH COMPRISES ADDING TO A MOLTEN BATH OF SUCH ALLOY 0.2 TO 0.7% BY WEIGHT OF A MIXTURE ESSENTIALLY CONSISTING OF 50 TO 75% OF A MIXTURE (1) OF 20 TO 40% OF A FINELY DIVIDED METALLIC COMPONENT SELECTED FROM THE GROUP CONSISTING OF AT LEAST ONE OF THE METALS MAGNESIUM AND ALUMINUM, 10 TO 40% OF SODIUM FLUORIDE AND 20 TO 70% OF SODIUM CARBONATE, AND 25 TO 50% OF A MIXTURE (2) OF 30 TO 60% OF SODIUM CHLORIDE AND 40 TO 70% OF POTASSIUM CHLORIDE. 